1. Field of the Invention
This invention relates to a rotary magnetic disk device, and more particularly to a rotary magnetic disk device used in a magnetic recording-reproducing apparatus for an electronic still camera and the like.
2. Description of the Prior Art
Recently developed electronic still camera systems utilize an image pickup device such as a solid state image sensing device or an image pickup tube combined with a recording device using an inexpensive magnetic disk as a recording medium. Such devices have a comparatively high memory capacity. In the operation of such camera systems, a subject is electronically still-photographed and recorded onto a rotating magnetic disk, and the reproduction of an image is carried out by a television, a printer or the like, which are provided separately.
The magnetic disk used in the above-described camera system is normally used in the form of a magnetic disk pack. The magnetic disk pack is rotatably provided therein with the magnetic disk for magnetically recording still image information and the like, and the magnetic disk pack is mounted for use with a feeder of a magnetic head assembled in an electronic camera.
In the image recording-reproducing system using the magnetic disk of the type described, a magnetic disk having a diameter of about 50 mm is used, on which there is formed a recording pattern of from 1 to 52 tracks from the outer periphery to the inner periphery of the magnetic disk. In the magnetic recording-reproducing apparatus of the type described, the feed pitch of the magnetic head is as low as about 100 .mu.m, which is far smaller than the feed pitch of an ordinary floppy disc. In consequence, the magnetic head should be moved with very high accuracy.
Heretofore, as the feeder of the magnetic head, an arrangement has been used wherein a head carriage having secured thereto the magnetic head is guided by guide bars and is threadably coupled to a lead screw, which lead screw is rotated to finely feed the magnetic head. However, if the lead screw were to be made with sufficiently high accuracy so as to satisfy the requirement for the feed of the magnetic head for recording image signals, the lead screw would become so expensive as to be impracticable.
Further, as another feeder of the magnetic head, there has been proposed an arrangement wherein the head carriage provided with the magnetic head is guided by the guide bars, one end of a steel belt fixed to the head carriage is wound around a pulley connected to an output shaft of a stepping motor in an .alpha. shape, and the other end of the steel belt is fixed to the head carriage. In this feeder the pulley is rotated to move the magnetic head. In order to finely feed the magnetic head, it is necessary to reduce the diameter of the pulley. However, if the diameter of the pulley is reduced, then the durability of the steel belt can be reduced. Furthermore, in this feeder of the magnetic head, in order to finely feed the magnetic head, it may be proposed to reduce an angle of driving of the stepping motor. However, there is a limit to this in practice. To finely feed the magnetic head, it may also be proposed to provide a speed reducer for obtaining a high reduction ratio between the pulley and the stepping motor. However, the speed reducer can be complicated in construction and the accuracy of feed can be reduced.
It is necessary to detect a movement value of the above-described magnetic head, and for this purpose, a movement value sensor for the magnetic head is provided. Heretofore, the movement value sensor for the magnetic head has comprised a slit plate formed with a plurality of slits at regular intervals. The slit plate is fixed to the head carriage provided with the magnetic head, and a movement value of the magnetic head is detected from a movement of the slit plate due to the movement of the head carriage. However, with such a conventional movement value sensor for the magnetic head, the feeder becomes disadvantageously complicated in construction.